In the manufacture of ceramic articles, it is common to employ a variety of polymers to act as binders to impart strength to the unfired "green" ceramic article. This is often accomplished by blending the ceramic powder with a binder solution containing polymers or by directly compounding the ceramic powder with molten polymers. The former method is generally used for tap casting, slip casting, and extrusion techniques whereas the latter is typically used in molding articles.
Both method suffer from a number of problems however. First, the extremely high viscosity of the polymeric solution restricts the concentration of ceramic powder which can be conveniently handled. This is particularly serious with sub-micron sized ceramic powders, especially those having a narrow particle size distribution. Secondly, when using the binder solution technique, the ceramic article has to be dried. Thirdly, it is difficult to completely remove the binder of either technique in the "binder burnout" stage prior to high temperature sintering.
The recent U.S. Pat. No. 4,587,068, issued for an invention of Borase et al. teaches a tape casting technique wherein the ceramic powder is mixed with a solution of monomers having a low vapor pressure. No solvents are present in the mixture. The tape is cast, then the monomer is polymerized. While there are some advantages to this method, the viscosity of the solvent-less fluid limits the applicability of the technique to tape casting or other molding methods involving a non-pourable paste. Further, due to the relatively high percentage of polymer in the article, a clean binder burnout may be difficult to achieve, particularly where the surface to volume ratio of the article is relatively small. Indeed, this patent itself notes that "some deformation of the ceramic material occurs during sintering." While Borase et al. do describe some relatively low viscosity slips at high solids loadings, e.g., 1180 cPs for 53.8 vol. % solids and 3500 cPs for 56.4 vol. % solids (although no shear rate is given; see discussion below), these are achieved only by extended milling times, respectively, of 90 hours (3.75 days) and 168 hours (7 days).
U.S. Pat. No. 3,962,162, issued for an invention of Schmank, discloses green refractory compositions including refractory powder, 1-30% unsaturated polyester, 1/2-12% unsaturated vinyl monomer, 0.2-0.5% catalyst for vinyl polymerization, 0.5-5% internal mold release compounds, and 0.5-7% volatile mold release and lubricant. The volatile mold release compounds described include butyl stearate, and other oily materials that volatilize without changing from about 120.degree. to 200.degree. C., including methyl stearate and dioctylphthalate. The mold release compound is then said to "be expelled" from solution during polymerization and appears to provide a reticulation of capillaries during volatilization so that subsequent firing can proceed without danger of rupture of the green ceramic during decomposition of polymerized binder."
These green composition prior to polymerization are variously described as "a fairly stiff plastic mass," "a very plastic dough," "a plastic dough," and "a stiff dough." These composition are also described as processed by extrusion at 300-700 psi through a die at elevated temperatures (130.degree.-140.degree. C.). Accordingly, these are not suitable for molding techniques requiring pourable slurries.
The addition of high aspect ratio particles (e.g., whiskers, fibers, platelets) to a material is one method of imparting greater fracture toughness, stiffness, and tensile strength to ceramic products. Whiskers are mono-crystalline materials with diameters that are typically less than 1 micrometer and lengths ranging from approximately 10-80 micrometers. Whiskers have been made from a number of materials including silicon carbide, silicon nitride, and aluminum oxide. Fibers are polycrystalline, generally 10-15 microns in diameter, and may range up to several centimeters long; they are typically composed of materials such as silicon carbide, graphite, borosilicate, boron nitride, aluminum oxide, and aluminum silicate. In order to impart optimum properties, the whiskers should be uniformly dispersed throughout the suspension. Due to particle-to-particle interactions, this has been difficult to achieve. Such interactions include both agglomerative effects (e.g., Van der Waals forces) and repulsive forces (e.g., electrostatic repulsion).
Particle surface area is also an important criteria. Large surface area associated with fine particles usually requires a lower concentration of dispersed phase. High surface area materials, in addition to fine, submicron particles, are typically high aspect ratio particles such as whiskers, fibers, and platelets, but can also include porous particles. Additions of even small quantities (e.g., 5% by volume) of such materials have a dramatic effect on the rheology of the system; in essence, the effective volume of the high surface area particles (i.e., the volume of any associated double layer and that subjected to steric and electrostatic neutralization necessary for dispersion) can be significantly greater than the actual particle volume. Unfortunately, whiskers and fibers are very useful for improving physical properties of ceramics, such as fracture toughness and strength; and such properties are best enhanced when the whiskers or fibers are uniformly dispersed in the ceramic matrix. Further, the higher the total solids loading (i.e., sinterable and non-sintered material, such as alumina and silicon carbide whiskers), the better the particle packing in the green state by which drying and associated problems are avoided, and hence a denser sintered article can eventually be obtained.
Not only are surface effects an obstacle to providing a homogeneous slurry (the precursor to a matrix having uniformly dispersed reinforcing particles), but chemically diverse particles are difficult to disperse. For example, it is difficult to mix partially stabilized colloidal zirconia with colloidal alumina in aqueous suspension. In conventional aqueous colloidal dispersions, the alumina particles (matrix phase) have a negative surface charge. When the partially stabilized zirconia (dopant), which as a positive surface charge, is added, the mixture hetero-coagulates. (The dopant is the material(s) in minor proportion to the matrix or main phase material; for whiskers and similar particles the dopant is typically unsintered in the final article, although some reaction may occur.) The use of dispersants helps, but is restricted due to the different surface chemistries of the colloidal phases. The coagulation imposes a rheological limit for the amount of dopant and matrix solids that can be added. Further, due to poor mixing, the resultant mixture is not homogeneous.